I would be very interested on the outcome of this...I was speaking with one of my consultants who I recommended get Vonage, and he called the other day and thought I was half-duplex also....I have 6 mbps down and 756 up also...

I think your experiencing latency, the time lag it takes for your voice to be packaged, unpackaged, moved to New Jersey and to the other end. If I test my Vonage phone by calling it with my landline phone sitting next to it when I say "Hello" it takes a 1/4 second to come out of the Vonage phone. Same thing going the other way. Testing it with my cell phone doubles that time. The problem seems to get better or worse depending on the Internet and who I am calling.

I don't know if there really is a cure. Good phone etiquette helps. ie. don't break into a sentence when the other person is speaking. Conference calls with 3 people don't work well at all for me. I live in the sticks at the bitter end of an Internet line. Their is no grid or redundancy here in Bend, Oregon. I think that makes it worse for me.

Please note that the above above is only a Snipe's eye view. I'm not a telephone engineer. If phones ran on live superheated steam I could probably pinpoint the problem

I've experienced that too. The best way I can describe it is that it sounds like your talking on a cell phone. You basically seem to "step" on the other person if your both talking at the same time. In my experience I can hang up and call them right back and it's fine.